The present invention relates to ongoing efforts to achieve effective delivery of functional nucleic acids to mammalian cells. More specifically, the invention relates to using bacterial minicell vectors to deliver functional nucleic acids to mammalian cells. The invention has particular utility for eliminating drug resistance, especially in the context of cancer and AIDS therapy, for promoting apoptosis and for countering neoplasticity in targeted cells.
Recent advances have highlighted a variety of techniques for introducing functional nucleic acids into cells. For example, liposome-based transfection methods can deliver exogenously produced nucleic acids. Such an exogenous approach has the drawback, however, of effecting only transient inhibition of a target. Additionally, liposomes are unstable in vivo. As an alternative to delivery of exogenously produced nucleic acids, vectors can deliver plasmids that encode functional nucleic acids, which are produced endogenously. The viral vectors currently useful for this purpose, however, poses serious safety concerns. Illustrative problems include recombination with wild-type viruses, insertional and oncogenic potential, virus-induced immunosuppression, limited capacity of the viral vectors to carry large segments of DNA, reversion to virulence of attenuated viruses, difficulties in recombinant virus manufacture and distribution, low stability, and adverse reactions, such as an inflammatory response, caused by existing immunity. An approach that obviated these problems would offer significant benefit in making delivery of functional nucleic acids safer and more effective.
An effective method of delivering functional nucleic acids would be particularly beneficial for reversing drug resistance. Mammalian cells employ a variety of biological processes to resist drugs, which poses a major obstacle to the successful treatment of cancer. Similarly, drug resistance limits the efficacy of HIV treatment, particularly to highly active antiretroviral therapy (HAART), which is based on a combination of nucleoside reverse transcriptase inhibitors (NRTIs) and protease inhibitors (PIs) or a non-nucleoside reverse transcriptase inhibitor (NNRTI).
Clinical tumor resistance to chemotherapy can be intrinsic or acquired. Intrinsic resistance exists at the time of diagnosis in tumors that fail to respond to first-line chemotherapy. Acquired resistance occurs in tumors that may respond well to initial treatment, but exhibit a resistant phenotype upon recurrence. Such tumors gain resistance both to previously used drugs and to new drugs, including drugs with different structures and mechanisms of action. The term MDR (multidrug resistance) describes this phenomenon in which tumor cells become cross-resistant to several structurally unrelated drugs after exposure to a single drug.
The mechanisms for multi-drug resistance are complex and multifactorial, owing largely to the high level of genomic instability and mutations in cancer cells. Exemplary mechanisms are drug inactivation, extrusion of drug by cell membrane pumps, decreased drug influx, mutations of drug targets and failure to initiate apoptosis (Bredel, 2001; Chen et al., 2001; White and McCubrey, 2001; Sun et al., 2001).
Drug extrusion is particularly common, and can result from over-expression of membrane-associated proteins that pump drugs from the intracellular to the extracellular environment. Such pumps often are members of the ATP-binding cassette (ABC) transporter superfamily (Doige et al., 1993). P-glycoprotein (Pgp) is one such example, and is a major contributor to MDR in a variety of cancer cells (Endicott et al., 1989; Litman et al., 2001). Other examples include the MDR-associated protein (MRP; Cole et al., 1992), breast cancer resistance protein (BCRP; Litman et al., 2000), and lung resistance-related protein (LRP; a major vault protein; Scheffer et al., 2000). Other multidrug transporter proteins also have been identified in cancer cells (Gottesman et al., 2002) and in pathogenic microorganisms (Van Bambeke et al., 2000).
Resistance to apoptosis (programmed cell death) of tumor cells induced by cytotoxic agents and radiation (Sellers and Fisher, 1999) is another common mechanism. This mechanism frequently involves over-expression of anti-apoptotic proteins, such as B-cell leukemia protein 2 (Bcl-2), Bcl-XL, Bcl-W, A1/Bfl1, Mcl-1 and mutations in the p53 protein. Although a precise understanding of how proteins like Bcl-2 exerts their anti-apoptotic effects remains elusive, the proteins are over-expressed in many cancers including colorectal, prostate, and breast cancers (Hanada, et al., 1995; Bakhshi et al., 1985; Wang et al., 1996). Increased expression of the transcription factor nuclear factor kappa B (NF-κB) also is a major mechanism for tumor cells to acquire chemotherapy resistance (Wang et al., 1999).
Drugs to counter MDR have been identified, such as drugs that block the action of P-glycoprotein (List et al., 1993; Miller et al., 1991; Wishart et al., 1992). Many such drugs were ineffective in clinical trials, however, because they bound to the plasma of patients, could not reach their destination (Ayesh et al., 1996a; Broxterman et al., 1987; Lehnert et al., 1996) and were toxic to normal cells. The use of functional nucleic acids to counter MDR also has been attempted. Yet, as noted above, existing vectors for this purpose are unstable or toxic, or they pose other serious safety issues, which hamper their use in humans (Sioud, 2004).
Accordingly, a continuing need exists for tools and methods for delivering functional nucleic acids that reduce drug resistance, promote apoptosis, and counter neoplasticity in target cells.